Equalized multi-signaling mode driver

ABSTRACT

A transmit circuit can be configured to output two-level pulse amplitude modulation (PAM-2) or four-level pulse amplitude modulation (PAM-4). In the PAM-2 mode, pre-tap feed-forward equalization (FFE) and post-tap FFE can be applied to the PAM-2 signal by pre-taps and post-taps, respectively. In the PAM-4 mode, at least one post-tap is repurposed to generate, along with the main tap, the main PAM-4 signaling levels. At least one PAM-2 FFE tap is repurposed to apply FFE in the PAM-4 mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a block diagram illustrating an embodiment of a multi-modesignaling system.

FIG. 2 is a block diagram illustrating a memory system.

FIG. 3A illustrates an equalized multi-signaling mode driver intwo-level pulse amplitude modulation (PAM-2) mode.

FIG. 3B illustrates an equalized multi-signaling mode driver infour-level pulse amplitude modulation (PAM-4) mode.

FIG. 4 illustrates a multi-signaling mode driver with arbitrary numbersof pre-tap FFE and post-tap FFE taps.

FIG. 5 illustrates an embodiment of a multi-signaling mode driver.

FIG. 6A illustrates a PAM-2 eye pattern without equalization.

FIG. 6B illustrates a PAM-2 eye pattern with equalization.

FIG. 7A illustrates a PAM-4 eye pattern without equalization.

FIG. 7B illustrates a PAM-4 eye pattern with equalization.

FIG. 8 is a flowchart illustrating a method of configuring amulti-signaling mode driver.

FIG. 9 is a flowchart illustrating a method of applying FFE to differentlevels of PAM signaling.

FIG. 10 is a block diagram of a computer system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A driver can be configured to use two-level pulse amplitude modulation(PAM-2) or four-level pulse amplitude modulation (PAM-4). In the PAM-2mode, feed-forward equalization (FFE) can be applied to the PAM-2 signalby pre-taps and post-taps, respectively. In the PAM-4 mode, at least oneFFE tap is repurposed to generate, along with the main tap, the mainPAM-4 signaling levels. In an embodiment, at least one PAM-2 tap can berepurposed to apply FFE in the PAM-4 mode. In another embodiment, noPAM-2 taps are repurposed to apply FFE in the PAM-4 mode.

FIG. 1 shows a block diagram illustrating an embodiment of a multi-modesignaling system. Signaling system 100 comprises a driving integratedcircuit, a receiving integrated circuit, and interconnect between them.The driving integrated circuit includes transmitter circuit 110 (a.k.a.,a driver). Transmitter circuit 110 uses finite impulse response (FIR)based equalization. Transmitter circuit 110 includes tap drivers 113.Tap drivers 113 may include a pre-tap FFE driver, a post-tap FFE driver,and a main tap driver.

The receiver integrated circuit includes receiver circuit 150. Theinterconnect between the driving integrated circuit and the receivingintegrated circuit comprises interconnect system 140. Interconnectsystem 140 would typically comprise a printed circuit (PC) board,connector, cable, flex circuit, other substrate, and/or a combination ofthese. Interconnect system 140 may be and/or include one or moretransmission lines.

Receiver circuit 150 would typically be part of an integrated circuitthat is receiving the signal sent by the driving integrated circuit. Itshould be understood that termination (not shown in FIG. 1) can be partof the integrated circuit or interconnect system 140. It should also beunderstood that although system 100 is illustrated as transmitting asingle-ended signal, the signals sent by the driving integrated circuitof system 100 may represent one of a pair of differential signals or oneof a collection of signals sending multi-wire-coded data.

In FIG. 1, the output of transmitter circuit 110 is connected to a firstend of interconnect system 140. The second end of interconnect system140 is connected to the input of receiver 150. Transmitter circuit 110can be configured to drive either PAM-2 or PAM-4 signaling levels. InPAM-2 mode, transmitter circuit 110 can use tap drivers 113 to applypre-tap FFE, post-tap FFE and drive the main (two-level) signal. InPAM-4 mode, transmitter circuit 110 can use tap drivers 113 to applypre-tap FFE, post-tap FFE and drive the main (four-level) signal.Transmitter circuit 110 may repurpose at least one tap driver 113 togenerate, along with the main tap, the main (four-level) signal ratherthan applying FFE. In an embodiment, a tap driver 113 that is used toapply post-tap FFE in PAM-2 mode is repurposed to generate, along withthe main tap, the main (four-level) signal in PAM-4 mode.

In an embodiment, a tap driver 113 that is used to apply pre-tap FFE inPAM-2 mode may be repurposed to apply post-tap FFE to the main(four-level) signal in PAM-4 mode. The application of this post-tap FFEto the main (four-level) signal in PAM-4 mode by this repurposed tapdriver 113 can be in addition to the repurposing of a tap driver 113 tohelp generate the main (four-level) signal in PAM-4 mode.

FIG. 2 is a block diagram illustrating a memory system. In FIG. 2,memory system 200 comprises memory controller 210 and memory 220. Memorycontroller 210 includes drivers 213 and receivers 214. Memory controller210 also includes N number of signal ports Q[1:N] that may be driven byone or more of drivers 213 and may receive signals to be sampled by oneor more of receivers 214. Memory 220 includes drivers 223 and receivers224. Memory 220 also includes N number of signal ports Q[1:N] that maybe driven by one or more of drivers 223 and may receive signals to besampled by one or more of receivers 224. Signal ports Q[1:N] of memorycontroller 210 are operatively coupled to ports Q[1:N] of memory 220,respectively. Receivers 224 of memory 220 may receive one or more of theQ[1:N] signals from memory controller 210. Receivers 214 of memorycontroller 210 may receive one or more of the Q[1:N] signals from memory220.

One or more of drivers 213 when configured and coupled with acorresponding one or more receivers 224 may form a PAM-2 signalingsystem or a PAM-4 signaling system. Thus, one or more of drivers 213 ofmemory controller 210 may correspond to transmitter circuit 110,discussed previously, or correspond to a transmitter circuit discussedherein subsequently. In these cases, the one or more of drivers 213 ofmemory controller 210 may, in PAM-2 mode, use its tap drivers to applypre-tap FFE, post-tap FFE, and drive the main (two-level) signal. InPAM-4 mode, the one or more of drivers 213 may use its tap drivers toapply pre-tap FFE, post-tap FFE, and drive the main (four-level) signal.Drivers 213 may repurpose at least one of its tap drivers to generate,along with the main tap, the main (four-level) signal rather thanapplying FFE. In an embodiment, a tap driver that is used to applypost-tap FFE in PAM-2 mode is repurposed by a driver 213 to generate,along with the main tap, the main (four-level) signal in PAM-4 mode. Atap driver of a driver 213 that is used to apply pre-tap FFE in PAM-2mode may be repurposed to apply post-tap FFE to the main (four-level)signal in PAM-4 mode. The application of this post-tap FFE to the main(four-level) signal in PAM-4 mode by this repurposed tap driver can bein addition to the repurposing of a tap driver to generate the main(four-level) signal in PAM-4 mode.

Likewise, one or more of drivers 223 when coupled with a correspondingone or more receivers 214 may form a PAM-2 signaling system or a PAM-4signaling system. Thus, one or more of drivers 223 of memory 220 maycorrespond to transmitter circuit 110, discussed previously, orcorrespond to a transmitter circuit discussed herein subsequently. Inthese cases, the one or more of drivers 223 of memory 220 may, in PAM-2mode, use its tap drivers to apply pre-tap FFE, post-tap FFE, and drivethe main (two-level) signal. In PAM-4 mode, the one or more of drivers223 may use its tap drivers to apply pre-tap FFE, post-tap FFE, anddrive the main (four-level) signal. Drivers 223 may repurpose at leastone of its tap drivers to generate, along with the main tap, the main(four-level) signal rather than applying FFE. In an embodiment, a tapdriver that is used to apply post-tap FFE in PAM-2 mode is repurposed bya driver 223 to generate, along with the main tap, the main (four-level)signal in PAM-4 mode. A tap driver of a driver 223 that is used to applypre-tap FFE in PAM-2 mode may be repurposed to apply post-tap FFE to themain (four-level) signal in PAM-4 mode. The application of this post-tapFFE to the main (four-level) signal in PAM-4 mode by this repurposed tapdriver can be in addition to the repurposing of a tap driver to helpgenerate the main (four-level) signal in PAM-4 mode.

Memory controller 210 and memory 220 are integrated circuit typedevices, such as one commonly referred to as a “chip”. A memorycontroller, such as memory controller 210, manages the flow of datagoing to and from memory devices, such as memory 220. For example, amemory controller may be a northbridge chip, an application specificintegrated circuit (ASIC) device, a graphics processor unit (GPU), asystem-on-chip (SoC) or an integrated circuit device that includes manycircuit blocks such as ones selected from graphics cores, processorcores, and MPEG encoder/decoders, etc. Memory 220 can include a dynamicrandom access memory (DRAM) core or other type of memory cores, forexample, static random access memory (SRAM) cores, or non-volatilememory cores such as flash. In addition, although the embodimentspresented herein describe memory controller and components, the instantapparatus and methods may also apply to chip interfaces that effectuatesignaling between separate integrated circuit devices.

It should be understood that signal ports Q[1:N] of both memorycontroller 210 and memory 220 may correspond to any input or output pins(or balls) of memory controller 210 or memory 220 that transmitinformation between memory controller 210 and memory 220. For example,signal ports Q[1:N] can correspond to bidirectional data pins (or padmeans) used to communicate read and write data between memory controller210 and memory 220. The data pins may also be referred to as “DQ” pins.Thus, for a memory 220 that reads and writes data up to 16 bits at atime, signal ports Q[1:N] can be seen as corresponding to pins DQ[0:15].In another example, signal ports Q[1:N] can correspond to one or moreunidirectional command/address (C/A) bus. Signal ports Q[1:N] cancorrespond to one or more unidirectional control pins. Thus, signalports Q[1:N] on memory controller 210 and memory 220 may correspond topins such as CS (chip select), a command interface that includes timingcontrol strobes such as RAS and CAS, address pins A[0:P] (i.e., addresspins carrying address bits), DQ[0:X] (i.e., data pins carrying databits), etc., and other pins in past, present, or future devices.

FIG. 3A illustrates an equalized multi-signaling mode driver intwo-level pulse amplitude modulation (PAM-2) mode. In FIG. 3A driver310, comprises FIR logic 330, mode logic 350, tap driver 314, tap driver315, and tap driver 316. FIR logic 330 includes shift register 331 andshift register 332. In the particular embodiment shown in FIGS. 3A and3B, shift register 331 and shift register 332 are three elements deep.Shift register 331 receives a data value D[0] that is to be transmittedin PAM-4 mode. Shift register 332 receives a data value D[1] to betransmitted by driver 310 in both the PAM-2 and the PAM-4 modes. InPAM-4 mode, D[1] is the most significant bit and D[0] is the leastsignificant bit of the two-bit value transmitted by driver 310. In PAM-2mode, D[1] is the one-bit value transmitted by driver 310. Mode logic350 receives an indicator that configures driver 310 to either drivePAM-2 signals or to drive PAM-4 signals. Driver 310 (and FIR logic 330,in particular) receives at least one timing reference, CK, to indicateand/or control the timing signals internal to, received by, and/oroutput by, driver 310.

Tap drivers 314-316 are each operatively coupled to mode logic 350. Tapdrivers 314-316 are each operatively coupled to mode logic 350 in orderto receive values to drive, and to receive tap drive output timing. Theoutputs of tap drivers 314-316 are added together to produce the signalat the output node of driver 310 (OUT). In an embodiment using currentmode type tap drivers 314-316 (e.g., open-drain), the outputs of tapdrivers 314-316 may be connected together in order to add the outputs oftap drivers 314-316 and produce the signal at the output node of driver310.

In an embodiment, tap drivers 314-316 each receive drive strengthvalues—referred to herein as tap weights. Tap driver 314 receives tapweight W[+1]. Tap driver 315 receives tap weight W[0]. Tap driver 314receives tap weight W[−1]. Each tap weight is supplied to a respectivetap driver 314-316 to control the level of the output signal generatedby the respective tap driver 314-316.

In PAM-2 mode and PAM-4 mode, shift register 332 is used to store apre-tap data value D₊₁, a primary data value D₀, and a post-tap datavalue D⁻¹. In PAM-2 mode, the primary data value of shift register 332is the data value to be transmitted during a given transmit interval. InPAM-4 mode, the primary data value is the most significant bit of thetwo-bit data value to be transmitted during a given transmit interval.The pre- and post-tap data values are the next-to-be transmitted andpreviously transmitted, respectively. In other words, the subscript ofthe “D” elements of shift register 332 indicates the number of transmitintervals that are to transpire before the data value stored in thatelement will be transmitted. In PAM-2 mode, data value D[0], andtherefore shift register 331, is not used.

In PAM-4 mode, shift register 331 is used to store a pre-tap data valueD₊₁, a primary data value D₀, and a post-tap data value D⁻¹. The primarydata value of shift register 331 is the least significant bit of thetwo-bit data value to be transmitted during a given transmit interval.The pre- and post-tap data values are the next-to-be transmitted andpreviously transmitted, respectively. In other words, the subscript ofthe “D” elements of shift register 331 indicates the number of transmitintervals that are to transpire before the data value stored in thatelement will be transmitted.

In FIG. 3A, mode logic 350 configures driver 310 to drive PAM-2 signallevels. Thus, as illustrated by arrow 352, the pre-tap data value D₊₁ ofshift register 332 is operatively coupled to, by mode logic 350, theinput of tap driver 314. As illustrated by arrow 351, the primary datavalue D₀ of shift register 332 is operatively coupled, by mode logic350, to the input of tap driver 315. And, as illustrated by arrow 353,the post-tap data value D⁻¹ of shift register 332 is operativelycoupled, by mode logic 350, to the input of tap driver 316. Accordingly,in PAM-2 mode, tap driver 314 functions as a pre-tap driver (i.e.applies pre-tap FFE); tap driver 315 functions as the primary (or main)tap driver; and, tap driver 316 functions as a post-tap driver (i.e.,applies post-tap FFE).

FIG. 3B illustrates an equalized multi-signaling mode driver infour-level pulse amplitude modulation (PAM-4) mode. In FIG. 3B, modelogic 350 configures driver 310 to drive PAM-4 signal levels. Asillustrated by arrow 355, the primary data value D₀ of shift register332 is operatively coupled, by mode logic 350, to the input of tapdriver 315. As illustrated by arrow 356, the primary data value D₀ ofshift register 331 is operatively coupled, by mode logic 350, to theinput of tap driver 316. As illustrated by arrow 357, the post-tap datavalue D⁻¹ of shift register 331 is operatively coupled, by mode logic350, to the input of tap driver 314. Likewise, as illustrated by arrow358, the post-tap data value D⁻¹ of shift register 332 is operativelycoupled, by mode logic 350, to the input of tap driver 314. Accordingly,in PAM-4 mode, tap driver 314 functions as a post-tap driver (i.e.,applies post-tap FFE); tap driver 315 functions as the primary (or main)tap driver for the most significant bit of the two-bit value beingdriven; and, tap driver 316 functions as the primary (or main) tapdriver for the least significant bit of the two-bit value being driven.

In an embodiment, the tap weights W[+1], W[0], and W[−1] used in PAM-4mode are the same tap weights used in PAM-2 mode. In another embodiment,tap weights W[0] applied to tap driver 315 and tap weight W[−1] appliedto tap driver 316 may be set to provide the appropriate drive strengthsto implement PAM-4 signaling levels and re-set to implement PAM-2signaling levels, as appropriate. For example, to implement PAM-4signaling levels, tap weight W[−1] may be set to approximately 50% oftap weight W[0] (e.g., if W[0]=2.0, then W[−1]=1.0.) This would providerelative signaling levels of: −3α, −α, +α, and +3α for D[1:0] values of00, 01, 10, and 11, respectively (where a is an arbitrary constant thatdetermines the magnitude of the PAM-4 signal).

In an embodiment, mode logic 350 also combines the post-tap values fromD⁻¹ of shift register 331 and D⁻¹ of shift register 332 (as illustratedby arrows 357 and 358) to produce a one bit equalization value that isinput to tap driver 314. Examples of the relative drive strengths(compared to other values in a column) output by tap driver 314 aregiven in Table 1. It should be understood that the columns labeled“2-bit EQ” and “1½-bit EQ” would require a two bit equalization value tobe input to tap driver 314, or an additional tap driver. It should alsobe understood that the column labeled “1-bit EQ” can be implementedusing the post-tap value from D⁻¹ of shift register 332 (as shown byarrow 358) without using the using the post-tap value from D⁻¹ of shiftregister 331. In an embodiment (not shown in FIG. 3B), the FFE appliedcan be pre-tap FFE instead of a post-tap FFE. In this embodiment, modelogic 350 will be configured such that tap driver 314 will receive itsinputs from the D₊₁ of shift register 331 and the D₊₁ of shift register332 (instead of the D⁻¹'s.)

TABLE 1 Previous 2-bit EQ 1-bit EQ 1½-bit EQ PAM-4 level (relativestrength) (relative strength) (relative strength)  3α 3 2 3 α 1 2 0 −α−1 −2 0 −3α −3 −2 −3

FIG. 4 illustrates a multi-signaling mode driver with arbitrary numbersof pre-tap FFE and post-tap FFE taps. In FIG. 4, driver 410, comprisesFIR logic 430, mode logic 450, a plurality of PAM-2 pre-tap FFE tapdrivers 413-414, main tap driver 415, and a plurality of PAM-2 post-tapFFE tap drivers 416-417. FIR logic 430 includes shift register 431 andshift register 432. In the particular embodiment illustrated in FIG. 4,shift register 431 and shift register 432 are an arbitrary number ofelements deep. Shift register 431 and shift register 432 are illustratedwith N number of pre-tap data values (D_(+N) to D₊₁), a primary datavalue D₀, and M number of post-tap data values (D⁻¹ to D_(−M).)

Shift register 431 receives a data value D[0] that is to be transmittedin PAM-4 mode. Shift register 432 receives a data value D[1] to betransmitted by driver 410 in both PAM-2 and PAM-4 modes. In PAM-4 mode,D[1] is the most significant bit and D[0] is the least significant bitof the two-bit value to be transmitted by driver 410. In PAM-2 mode,D[1] is the one-bit value to be transmitted by driver 410. Mode logic450 receives an indicator that configures driver 410 to either drivePAM-2 signals or to drive PAM-4 signals. Driver 410 (and FIR logic 430,in particular) receives at least one timing reference, CK, to indicateand/or control the timing signals internal to, received by, and/oroutput by, driver 410.

Tap drivers 413-417 are each operatively coupled to mode logic 450. Tapdrivers 414-417 are each operatively coupled to mode logic 450 in orderto receive values to drive, and to receive tap drive timing. The outputsof tap drivers 413-417 are added together to produce the signal at theoutput node of driver 410 (OUT). In an embodiment using current modetype tap drivers 413-417 (e.g., open-drain), the outputs of tap drivers413-417 may be connected together in order to add the outputs of tapdrivers 413-417 and produce the signal at the output node of driver 410.

In an embodiment, tap drivers 413-417 each receive tap weights. Tapdriver 413 receives tap weight W[+N], the next tap driver (not shown inFIG. 4) receives tap weight W[N−1], and so on, to tap driver 414 whichreceives tap weight W[+1], tap. Tap driver 315 receives tap weight W[0].Tap driver 416 receives tap weight W[−1], the next tap driver (not shownin FIG. 4) receives tap weight W[−2], and so on, to tap driver 417 whichreceives tap weight W[−M]. Each tap weight is supplied to a respectivetap driver 413-417 to control the level of the output signal generatedby the respective tap driver 413-417.

In PAM-2 mode and PAM-4 mode, shift register 432 is used to storepre-tap data values D_(+N) through D₊₁, a primary data value D₀, andpost-tap data values D⁻¹ through D_(−M). In PAM-2 mode, the primary datavalue is the data value to be transmitted during a given transmitinterval. In PAM-4 mode, the primary data value of shift register 432 isthe most significant bit of the two-bit data value to be transmittedduring a given transmit interval. The subscript of the “D” elements ofshift register 432 indicates the number of transmit intervals that areto transpire before the data value stored in that element will betransmitted. In PAM-2 mode, data value D[0], and therefore shiftregister 431, is not used

In PAM-4 mode, shift register 431 is used to store pre-tap data valuesD_(+N) through D₊₁, a primary data value D₀, and post-tap data valuesD⁻¹ through D_(−M). The primary data value of shift register 431 is theleast significant bit of the two-bit data value to be transmitted duringa given transmit interval. The subscript of the “D” elements of shiftregister 431 indicates the number of transmit intervals that are totranspire before the data value stored in that element will betransmitted.

In PAM-2 mode, mode logic 450 configures driver 410 to drive PAM-2signal levels. Accordingly, in PAM-2 mode, tap drivers 413-414 functionas pre-tap drivers (i.e., tap drivers 413-414 apply pre-tap FFE); tapdriver 315 functions as the primary (or main) tap driver; and, tapdrivers 416-417 function as a post-tap drivers (i.e., tap drivers416-417 apply post-tap FFE.)

In PAM-4 mode, mode logic 450 configures driver 410 to drive PAM-2signal levels. In PAM-4 mode, one or more of tap drivers 413-414 and/ortap drivers 416-417 function as part of the primary (or main) tapdriver. The remaining tap drivers 413-414, and/or tap drivers 416-417,can be configured to be disabled, function as a pre-tap driver (i.e.,apply pre-tap FFE), or function as a post-tap driver (i.e., applypost-tap FFE.) In addition, in an embodiment, for higher order PAM modes(e.g., PAM-Q where Q is an integer greater than 4), an appropriatenumber (e.g., 3 for PAM-8) of tap drivers 413-414, and/or tap drivers416-417, can be configured to function as part of the primary (or main)tap driver with the remaining tap drivers disabled or at least one ofthe remaining tap drivers configured to function as a pre-tap driver, orfunction as a post-tap driver.

FIG. 5 illustrates an embodiment of a multi-signaling mode driver. InFIG. 5, driver 500 comprises tap driver 514, tap driver 515, tap driver516, latch 532, latch 533, latch 534, latch 535, latch 536, 2:1multiplexor (MUX) 554, 2:1 MUX 555, 2:1 MUX 556, and PAM-4 equalizationlogic 557. A timing reference (CK) is received by PAM-4 equalizationlogic 557 and at each of the clock inputs of latches 532-534. Latch 532receives data value D[1]. The output of latch 532 is connected to theinput of latch 533 and the “0” input of MUX 554. The output of latch 533is connected to PAM-4 equalization logic 557, the input of latch 534,and the “0” input of MUX 555. The output of latch 534 is connected tothe “0” input of MUX 556. It should be understood that the term ‘latch’as used herein refers to a clocked data storage element and thereforeincludes (but is not limited to) the use of flip-flops, master-slave,and transparent latches, as appropriate, for latches 532-536.

Latch 535 receives data value D[0]. The output of latch 535 is connectedto the input of latch 536. The output of latch 536 is connected to PAM-4equalization logic 557 and the “1” input of MUX 556. The output of PAM-4equalization logic 557 is connected to the “1” input of MUX 554.

The output of MUX 554 is connected to the input of tap driver 514. Theoutput of MUX 555 is connected to the input of tap driver 515. Theoutput of MUX 556 is connected to the input of tap driver 516. The modeindicator signal (MODE) is connected to the control inputs of MUX 554and MUX 556. The control input of MUX 555 receives a constant logiczero. Thus, MUX 555 always passes the value at its “0” input (from theoutput of latch 533) to the output of MUX 555 (i.e., the input of tapdriver 515.) Tap driver 514 receives tap weight W[+1]. Tap driver 515receives tap weight W[0]. Tap driver 516 receives tap weight W[−1]. Theoutputs of tap drivers 514-516 are added together to produce the signalat the output node of driver 500 (OUT). In an embodiment using currentmode type tap drivers 514-516 (e.g., open-drain), the outputs of tapdrivers 514-516 may be connected together in order to add the outputs oftap drivers 514-516 and produce the signal at the output node of driver500.

In PAM-2 mode (i.e., MODE=0), tap driver 514 functions to apply pre-tapFFE (i.e., as a pre-tap) and tap driver 516 functions to apply post-tapFFE (i.e., as a post-tap). Tap driver 515 functions as the main tap toprovide the primary part of the PAM-2 signal. In PAM-4 mode (i.e.,MODE=1), tap driver 516 functions as part of the main tap to provide theleast significant bit part of the PAM-4 signal to the output node, OUT.Tap driver 515 functions as part of the main tap to provide the mostsignificant bit part of the PAM-4 signal to the output node, OUT. Tapdriver 514 functions to apply post-tap FFE as controlled by PAM-4equalization logic 557. It should be understood that, in PAM-4 mode,driver 500 is applying 1-bit post-tap FFE (see, e.g., Table 1) to thePAM-4 signal. It should also be understood that the overhead added to adriver 500 in order to have it function in both PAM-2 and PAM-4 modes islatch 535, latch 536, 2:1 multiplexor (MUX) 554, 2:1 MUX 555, 2:1 MUX556, and PAM-4 equalization logic 557.

FIG. 6A illustrates a PAM-2 eye pattern without equalization. FIG. 6Billustrates a PAM-2 eye pattern with equalization. As is illustrated bycomparing FIG. 6A to FIG. 6B, applying equalization (pre-tap andpost-tap) improves the characteristics of the PAM-2 eye pattern.

FIG. 7A illustrates a PAM-4 eye pattern without equalization. FIG. 7Billustrates a PAM-4 eye pattern with one bit of equalization. As isillustrated by comparing FIG. 7A to FIG. 7B, applying a one bit post-tapFFE equalization improves the characteristics of the PAM-4 eye pattern.

FIG. 8 is a flowchart illustrating a method of configuring amulti-signaling mode driver. The steps illustrated in FIG. 8 may beperformed by one or more elements of signaling system 100, memory system200, driver 310, driver 410, and/or driver 500. A transmit circuit witha main tap is configured to generate a PAM-2 signal (802). For example,driver 310 may be configured to generate a PAM-2 signal (as illustratedin FIG. 3A) using tap driver 315 as a main tap.

A first FFE tap of the transmit circuit is configured to apply pre-tapFFE to the PAM-2 signal (804). For example, tap driver 314 may beconfigured, by mode logic 350, to apply pre-tap FFE to the primarysignal generated by tap driver 315. A second FFE tap of the transmitcircuit is configured to apply post-tap FFE to the PAM-2 signal (806).For example, tap driver 316 may be configured, by mode logic 350, toapply post-tap FFE to the primary signal generated by tap driver 315.

The main tap and the second FFE tap are configured to generate a PAM-4signal (808). For example, mode logic 350 may configure tap driver 315and tap driver 316 to generate the four signal levels of PAM-4signaling. Tap driver 315 may generate the portion of the PAM-4 signalassociated with most significant bit being transmitted. Tap driver 316may generate the portion of the PAM-4 signal associated with leastsignificant bit being transmitted.

The first FFE tap is configured to apply post-tap FFE to the PAM-4signal (810). For example, mode logic 350 may configure tap driver 314to apply post-tap FFE to the PAM-4 signal generated by the combinationof tap driver 315 and tap driver 316.

FIG. 9 is a flowchart illustrating a method of configuring amulti-signaling mode driver. The steps illustrated in FIG. 9 may beperformed by one or more elements of signaling system 100, memory system200, driver 310, driver 410, and/or driver 500. A main tap of a transmitcircuit with a plurality of FFE taps is configured to generate a PAM-2signal (902). For example, driver 410 (with FFE tap drivers 413-414 and416-417) may be configured to generate a PAM-2 signal with tap driver415 functioning as a main tap.

At least one of the plurality of FFE taps is configured to apply pre-tapFFE to the PAM-2 signal (904). For example, one or more of FFE tapdrivers 413-414 may be configured by mode logic 450 to apply pre-tap FFEto the PAM-2 signal generated by tap driver 415. At least one of theplurality of FFE taps is configured to apply post-tap FFE to the PAM-2signal (906). For example, one or more of FFE tap drivers 416-417 may beconfigured by mode logic 450 to apply post-tap FFE to the PAM-2 signalgenerated by tap driver 415.

The main tap and at least one of the plurality of FFE taps areconfigured to generate a PAM-4 signal (908). For example, tap driver 415and one or more of tap drivers 416-417 may be configured by mode logic450 to generate the signal levels for PAM-4 signaling. At least one ofthe plurality of FFE taps are configured to apply post-tap FFE and/orpre-tap FFE to the PAM-4 signal. For example, one or more of tap drivers413-414 and any remaining tap drivers 416-417 may be configured to applypost-tap FFE and/or pre-tap FFE to the PAM-4 signal.

The methods, systems and devices described above may be implemented incomputer systems, or stored by computer systems. The methods describedabove may also be stored on a non-transitory computer readable medium.Devices, circuits, and systems described herein may be implemented usingcomputer-aided design tools available in the art, and embodied bycomputer-readable files containing software descriptions of suchcircuits. This includes, but is not limited to one or more elements ofsignaling system 100, memory system 200, driver 310, driver 410, and/ordriver 500, and their components. These software descriptions may be:behavioral, register transfer, logic component, transistor, and layoutgeometry-level descriptions. Moreover, the software descriptions may bestored on storage media or communicated by carrier waves.

Data formats in which such descriptions may be implemented include, butare not limited to: formats supporting behavioral languages like C,formats supporting register transfer level (RTL) languages like Verilogand VHDL, formats supporting geometry description languages (such asGDSII, GDSIII, GDSIV, CIF, and MEBES), and other suitable formats andlanguages. Moreover, data transfers of such files on machine-readablemedia may be done electronically over the diverse media on the Internetor, for example, via email. Note that physical files may be implementedon machine-readable media such as: 4 mm magnetic tape, 8 mm magnetictape, 3½ inch floppy media, CDs, DVDs, and so on.

FIG. 10 illustrates a block diagram of a computer system. Computersystem 1000 includes communication interface 1020, processing system1030, storage system 1040, and user interface 1060. Processing system1030 is operatively coupled to storage system 1040. Storage system 1040stores software 1050 and data 1070. Processing system 1030 isoperatively coupled to communication interface 1020 and user interface1060. Computer system 1000 may comprise a programmed general-purposecomputer. Computer system 1000 may include a microprocessor. Computersystem 1000 may comprise programmable or special purpose circuitry.Computer system 1000 may be distributed among multiple devices,processors, storage, and/or interfaces that together comprise elements1020-1070.

Communication interface 1020 may comprise a network interface, modem,port, bus, link, transceiver, or other communication device.Communication interface 1020 may be distributed among multiplecommunication devices. Processing system 1030 may comprise amicroprocessor, microcontroller, logic circuit, or other processingdevice. Processing system 1030 may be distributed among multipleprocessing devices. User interface 1060 may comprise a keyboard, mouse,voice recognition interface, microphone and speakers, graphical display,touch screen, or other type of user interface device. User interface1060 may be distributed among multiple interface devices. Storage system1040 may comprise a disk, tape, integrated circuit, RAM, ROM, EEPROM,flash memory, network storage, server, or other memory function. Storagesystem 1040 may include computer readable medium. Storage system 1040may be distributed among multiple memory devices.

Processing system 1030 retrieves and executes software 1050 from storagesystem 1040. Processing system 1030 may retrieve and store data 1070.Processing system 1030 may also retrieve and store data viacommunication interface 1020. Processing system 1050 may create ormodify software 1050 or data 1070 to achieve a tangible result.Processing system may control communication interface 1020 or userinterface 1060 to achieve a tangible result. Processing system 1030 mayretrieve and execute remotely stored software via communicationinterface 1020.

Software 1050 and remotely stored software may comprise an operatingsystem, utilities, drivers, networking software, and other softwaretypically executed by a computer system. Software 1050 may comprise anapplication program, applet, firmware, or other form of machine-readableprocessing instructions typically executed by a computer system. Whenexecuted by processing system 1030, software 1050 or remotely storedsoftware may direct computer system 1000 to operate as described herein.

The foregoing description of the invention has been presented forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed, andother modifications and variations may be possible in light of the aboveteachings. The embodiment was chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and various modifications as are suited to theparticular use contemplated. It is intended that the appended claims beconstrued to include other alternative embodiments of the inventionexcept insofar as limited by the prior art.

What is claimed is:
 1. An integrated circuit, comprising: a transmitcircuit having a multi-tap finite impulse response (FIR) equalizerincluding at least a main tap and a plurality of feed-forwardequalization (FFE) taps; and, mode selection circuitry to, in a twolevel pulse amplitude modulation (PAM-2) mode, configure the main tap togenerate a PAM-2 data signal, and to configure at least one of theplurality of FFE taps to apply FFE to the PAM-2 data signal, the modeselection circuitry also to, in a four level pulse amplitude modulation(PAM-4) mode, configure the main tap and at least one of the pluralityof FFE taps to generate a PAM-4 data signal.
 2. The integrated circuitof claim 1, wherein the mode selection circuitry is also to, in saidPAM-4 mode, configure at least one of the plurality of FFE taps to applypost-tap FFE to said PAM-4 data signal.
 3. The integrated circuit ofclaim 2, wherein said plurality of FFE taps comprises a first tap and asecond tap, the first tap, in the PAM-2 mode, is to be configured toapply pre-tap FFE to the PAM-2 data signal.
 4. The integrated circuit ofclaim 3, wherein the first tap, in the PAM-4 mode, is to be configuredto apply post-tap FFE to the PAM-4 data signal.
 5. The integratedcircuit of claim 3, wherein the second tap, in the PAM-2 mode, is to beconfigured to apply post-tap FFE to the PAM-2 data signal.
 6. Theintegrated circuit of claim 5, wherein, in the PAM-4 mode, the main tapis to be configured to drive a most significant bit of the PAM-4 datasignal and the second tap is to be configured to drive a leastsignificant bit of the PAM-4 data signal.
 7. The integrated circuit ofclaim 2, wherein said plurality of FFE taps comprises a first tap and asecond tap, wherein, in the PAM-2 mode, the first tap is to beconfigured to apply pre-tap FFE to the PAM-2 data signal and the secondtap is to be configured to apply post-tap FFE to the PAM-2 data signal.8. The integrated circuit of claim 2, wherein said plurality of FFE tapscomprises a first tap and a plurality of taps, wherein, in the PAM-2mode, the first tap is to be configured to apply post-tap FFE to thePAM-2 data signal and the plurality of taps is to be configured to applypre-tap FFE to the PAM-2 data signal.
 9. A transmitter that isselectively reconfigurable, comprising: a main tap to drive a two levelpulse amplitude modulation (PAM-2) signal; a first at least one tap toapply pre-tap feed-forward equalization (FFE) to said PAM-2 signal in afirst selectable configuration of the transmitter; and, a second atleast one tap to apply post-tap FFE to said PAM-2 signal in said firstselectable configuration of the transmitter, at least one of said firstat least one tap and said second at least one tap are reconfigurable ina second selectable configuration of the transmitter to drive a firstbit of a four level pulse amplitude modulation (PAM-4) signal.
 10. Thetransmitter of claim 9, wherein said second at least one tap isreconfigured in said second selectable configuration of the transmitterto drive said first bit said PAM-4 signal.
 11. The transmitter of claim10, wherein said first bit of said PAM-4 signal is a least significantbit.
 12. The transmitter of claim 11, wherein said main tap is to drivea most significant bit of said PAM-4 signal in said second selectableconfiguration of the transmitter.
 13. The transmitter of claim 9,wherein said first at least one tap is reconfigured in said secondselectable configuration of the transmitter to drive said first bit saidPAM-4 signal.
 14. The transmitter of claim 13, wherein said first bit ofsaid PAM-4 signal is a least significant bit.
 15. The transmitter ofclaim 14, wherein said main tap is to drive a most significant bit ofsaid PAM-4 signal in said second selectable configuration of thetransmitter.
 16. An integrated circuit, comprising: a selectivelyreconfigurable transmitter for selectively driving, with feed-forwardequalization (FFE), one of a PAM-2 and a PAM-4 signal, comprising: amain tap to drive a PAM-2 signal in a first configuration and a firstbit of a PAM-4 signal in a second configuration; and, a plurality oftaps to apply pre-tap FFE and post-tap FFE in said first configurationand to drive a second bit of said PAM-4 signal in said secondconfiguration.
 17. The integrated circuit of claim 16, wherein saidplurality of taps comprises a first set of taps and a second set oftaps, said first set of taps to apply said pre-tap FFE in said firstconfiguration and said second set of taps to apply said post-tap FFE insaid first configuration.
 18. The integrated circuit of claim 17,wherein said first set of taps apply post-tap FFE in said secondconfiguration.
 19. The integrated circuit of claim 17, wherein saidsecond set of taps to drive said second bit of said PAM-4 signal in saidsecond configuration.
 20. The integrated circuit of claim 16, whereinsaid plurality of taps comprises a first set of taps and a second set oftaps, said first set of taps to apply said pre-tap FFE in said firstconfiguration and said second set of taps to apply said post-tap FFE insaid first configuration, said first set of taps apply post-tap FFE insaid second configuration, said second set of taps to drive said secondbit of said PAM-4 signal in said second configuration.